In inverter control, bootstrap systems are mainly used to supply a power supply for a high-side gate driver from a power supply for a low-side gate driver. In the case where an inductive load is driven by an inverter circuit using MOSFETs, a freewheeling current of the inductive load flows back into a MOSFET, and the drain potential, which is the potential at a high-side main electrode terminal, becomes lower than the source potential, which is the potential at a low-side main electrode terminal. In particular, in the case where the freewheeling current flows through the low-side MOSFET, the drain potential thereof being lower than the source potential causes a bootstrap capacitor having one end connected to the drain terminal to be overcharged. The overcharge of the bootstrap capacitor may cause overvoltage in the gate driver or the MOSFET. In particular, in the case where an excessive principal current triggers protective operation to turn off the low-side MOSFET and cause a freewheeling current to flow, the freewheeling current also becomes a relatively large current. Accordingly, this problem becomes significant.
Patent literature 1 discloses a technique for solving this problem. In Patent literature 1, a half-bridge power circuit using GaN FETs as switching elements includes a bootstrap capacitor clamp circuit installed in series with a charging path of a bootstrap capacitor. Accordingly, even if the potential on the switching element side of the bootstrap capacitor becomes an excessively negative potential, control can be performed so that the voltage to which the bootstrap capacitor is charged may be constant.